SAN JUAN, Puerto Rico—Islands in the northern Caribbean braced Tuesday for the storm that could be Fred. The National Hurricane Center issued tropical storm warnings for the U.S. Virgin Islands and Puerto Rico, where forecasters expected the potential cyclone to strengthen Tuesday into the sixth named storm of the Atlantic hurricane season. “The most important thing today is preparation,” said Puerto Rico Gov. Pedro Pierluisi. “I am not going to minimize the potential impact of this event…we expect a lot of rain.” He said government agencies will close by noon on Tuesday. The Dominican Republic, Haiti and the southern Bahamas could get hit on Wednesday, and people in Florida were urged to monitor updates, but forecasters said it remained uncertain where the storm could move by later in the week. Heavy rains hit parts of the eastern Caribbean overnight, including the islands of Guadeloupe and Dominica. More than a month …